1. Field of the Invention
The present invention relates to a water electrolysis system including a water electrolysis apparatus for electrically decomposing water to generate oxygen and high-pressure hydrogen having a pressure higher than the oxygen, and a gas-liquid separator connected to a hydrogen pipe which discharges the high-pressure hydrogen from the water electrolysis apparatus, for separating water contained in the high-pressure hydrogen, and a method of operating such a water electrolysis apparatus.
2. Description of the Related Art
Generally, hydrogen is used as a fuel gas in a reaction to generate electric energy in fuel cells. The hydrogen is generated by water electrolysis apparatus, for example. A water electrolysis apparatus incorporates a solid polymer electrolyte membrane (ion exchange membrane) for electrically decomposing water to generate hydrogen (and oxygen). Electrode catalyst layers are disposed on the respective sides of the solid polymer electrolyte membrane, making up a membrane electrode assembly. Current collectors are disposed on the respective opposite sides of the membrane electrode assembly, making up a unit cell.
A plurality of such unit cells are stacked into a cell unit, and a voltage is applied across the cell unit while water is supplied to the current collectors on the anode side. On the anodes of the membrane electrode assembly, the water is decomposed to produce hydrogen ions (protons). The hydrogen ions permeate through the solid polymer electrolyte membranes to the cathodes, where the hydrogen ions combine with electrons to generate hydrogen. On the anodes, oxygen generated together with hydrogen is discharged with excess water from the cell units.
The water electrolysis apparatus generates hydrogen containing water. The water needs to be removed from the hydrogen to obtain hydrogen in a dry state, hereinafter referred to as “dry hydrogen”, which contains 5 ppm or less of water.
A differential-pressure-type high-pressure hydrogen manufacturing apparatus generates hydrogen under a high pressure, e.g., 1 MPa or higher, which is higher than oxygen, at cathodes. Such a differential-pressure-type high-pressure hydrogen manufacturing apparatus needs a large-size gas-liquid separator for removing water from high-pressure hydrogen.
There is known a gas-liquid separator disclosed in Japanese Laid-Open Patent Publication No. 2006-347779, for example. As shown in FIG. 24 of the accompanying drawings, the disclosed gas-liquid separator includes a pressure-resistant vessel 2 to which a hydrogen conduit 1 is connected, a water level sensor 3 for detecting a water level in the pressure-resistant vessel 2, a hydrogen removal conduit 4a of a hydrogen removal means 4 connected to a top wall of the pressure-resistant vessel 2, and a water drainage conduit (water drainage line) 5a of a water drainage means 5 connected to a bottom wall of the pressure-resistant vessel 2.
The hydrogen removal conduit 4a has a first back-pressure valve 6 and a solenoid-operated valve 7 disposed downstream of the first back-pressure valve 6. The water drainage conduit 5a has a second back-pressure valve 8.
The first back-pressure valve 6 is opened when the pressure of a fluid flowing thereinto reaches 35 MPa, for example. The second back-pressure valve 8 is opened when the pressure of a fluid flowing thereinto reaches a pressure level higher than the first back-pressure valve 6, e.g., 36 MPa. The solenoid-operated valve 7 is actuated in response to a detected signal from the water level sensor 3. Specifically, when the water level detected by the water level sensor 3 reaches a certain low level, the solenoid-operated valve 7 is opened, and when the water level detected by the water level sensor 3 reaches a certain high level, the solenoid-operated valve 7 is closed.
When the solenoid-operated valve 7 is closed, a high-pressure hydrogen gas removed from the pressure-resistant vessel 2 through the hydrogen removal conduit 4a is forcibly blocked thereby, causing the pressure in the pressure-resistant vessel 2 to increase beyond 35 MPa, i.e., the pressure setting of the first back-pressure valve 6. As a result, each time the pressure in the pressure-resistant vessel 2 reaches 36 MPa, i.e., the pressure setting of the second back-pressure valve 8, the second back-pressure valve 8 is opened. Consequently, water in liquid phase is intermittently discharged from the pressure-resistant vessel 2 through the water drainage conduit 5a and the second back-pressure valve 8.